1. Field of the Invention
The present invention relates to a solid oxide fuel cell, and more particularly to a solid oxide fuel cell for generating electricity by reacting fuel gas, reformed by a reformer, with air.
2. Description of the Related Art
In recent years various fuel cell devices fitted with fuel cells capable of generating electricity using fuel (hydrogen gas) and air, along with ancillary equipment for operating such fuel cells, have been proposed as next-generation energy sources.
Published Unexamined Application 2008-53209 (Patent Document 1) sets forth a fuel cell device for the steam reforming of fuel using a reformer, and for generating electricity using the steam gas thus obtained.
Prior Art References—Patent References: Patent Document 1: Published Unexamined Application 2008-53209.
As further detailed below, when a fuel cell device starts up, fuel and reforming air are first supplied into a reformer to conduct a partial oxidation reforming reaction (POX), fuel gas, reforming air, and water (pure water) are then supplied into the reformer, and an auto-thermal reforming reaction (ATR) proceeds, co-utilizing a partial oxidation reforming reaction (POX) and a steam reforming reaction (SR), described below; subsequently, fuel gas and water (pure water) are supplied into the reformer to conduct a steam reforming reaction (SR).
Within the fuel cell device, when the partial oxidation reforming reaction (POX) and auto-thermal reforming reaction (ATR) are implemented, reforming air is supplied to the reformer as fuel gas is supplied to the reformer.
At this point, the amount of fuel actually introduced into the reformer fluctuates depending on the pressure inside the reformer, even if the fuel supply device supplying fuel gas to the reformer is controlled at a fixed level. In a state whereby air is being introduced together with fuel gas into the reformer, the pressure inside the reformer rises as a result of the introduction of that air, making it difficult to introduce fuel gas into the reformer. This poses the problem that the targeted amount of fuel required to establish an appropriate level of reforming within the reformer may vary from the amount of fuel actually supplied. When the amount of fuel actually supplied varies from the target value, reforming air and fuel gas become unbalanced; carbon is deposited inside the reformer, and a dramatic speed up in the degradation of the reformer catalyst results.
It is possible to mitigate this type of pressure rise inside the reformer by increasing the volume of the reformer, but when the reformer increases in size it becomes difficult to heat uniformly, leading to thermal unevenness in the reformer. Enlarging the reformer also creates a further problem in that the overall size of the fuel cell device increases.
A more powerful fuel supply device for supplying fuel gas into the reformer could be used to reduce the effect of internal pressure within the reformer on fuel supply quantity, so that fuel gas could be fed in at a pressure which overcomes the pressure rise within the reformer, but increasing the power of the fuel supply device increases the size of the fuel supply device and raises cost.
The present invention was undertaken to resolve the above-described problems, and has the object of providing a solid oxide fuel cell capable of supplying an appropriate amount of fuel gas while achieving a smaller reformer size.
When conducting auto-thermal reforming reactions (ATR) and steam reforming reactions (SR) in a fuel cell device, water (pure water) must be supplied to the reformer as described above; in the ATR region, especially, where the amount of water supplied is extremely minute, only a very small amount of water, on the scale of a few milliliters per minute, must be continuously supplied. To precisely and stably supply such an extremely small amount of water requires the use of a specialized pump. The idea has been conceived of using a pulse pump to intermittently jet water under pulse control to supply such small amounts of water.
During steam reforming, however, the supplied water vaporizes inside the reformer, and the sudden increase in volume from that vaporization causes pressure fluctuations inside the reformer. Note that a vaporizer within the reformer vaporizes the supplied water, and fuel gas is reformed inside the reformer by the mixing of vaporized water and fuel gas. The vaporizer is here connected to the portion inside the reformer in which the reforming takes place, therefore the rising pressure within the vaporizer caused by the vaporization of water affects the entire reformer interior. The same phenomenon occurs even when the reformer and the vaporizer are constituted as separate entities.
A fuel-cell device is generally controlled so that the actual fuel supply quantity is measured and fed back to assure that the amount of fuel being supplied conforms to the target fuel supply quantity. If that feedback gain is raised simply to increase tracking performance relative to the targeted fuel supply quantity, the problem arises that fuel supply quantity rises and falls in an unstable manner in response to sudden pressure changes caused by the intermittent supply of water into the reformer
In other words, immediately after a fuel supply quantity detection sensor detects a state in which the amount of fuel gas supplied is less than the target value when pressure is rising, a determination is made that there is insufficient fuel gas, and control is executed whereby additional fuel gas is supplied. In actuality however, the pressure drops in the next instant, making fuel gas easier to supply, so the above-described fuel insufficiency is additionally supplied although it is in fact unneeded, resulting in an oversupply of fuel gas.
The present invention has the object of providing a solid oxide fuel cell capable of supplying an appropriate amount of fuel gas in a stable manner even when pressure fluctuations occur within the reformer as the result of the above-described intermittent introduction of water.